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Key Message

This indicator and its threshold values are based on a relatively limited data set for the grey seal. 
The indicator evaluation is carried out based on current data and methodological approaches, and applied to the whole Baltic Sea scale since the grey seal is known to disperse widely in the region. Work is required and underway for improvements and expansion of the data set to gain a greater spatial coverage.

 

This core indicator evaluates the status of the marine environment in terms of the nutritional status of seals, measured as average blubber thickness of seal populations. This signals both long term and short-term changes in food supply and many other stressors. Good status is achieved when the subcutaneous blubber thickness is above the defined threshold value, which reflects good conditions. In the current assessment (2011-2016) the grey seal failed to achieve the threshold value and the population is thus in not good status for the whole Baltic Sea.

 

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Key message figure 1. Status assessment results based on evaluation of the indicator 'nutritional status of seals' 2011-2016. The assessment is carried out using aggregated Scale 2 HELCOM assessment units – whole Baltic Sea excluding the Kattegat and Limfjord (defined in the HELCOM Monitoring and Assessment Strategy Annex 4). Status evaluation is carried out based on blubber thickness in grey seals. Click here to access interactive maps at the HELCOM Map and Data Service: nutritional status of seals.

 

Grey seals occur in the entire Baltic Sea, though in the Kattegat the species is rare and has not been breeding since the 1930s, except for a few observations in recent years. The status of the grey seal in the Baltic Sea is evaluated as a single unit, excluding the Kattegat which is evaluated separately. Grey seals do not achieve the threshold value with regard to nutritional status when evaluated as one single population at the scale of the entire Baltic Sea.

Ringed seals occur in the Gulf of Bothnia (northern management unit), and the Gulf of Finland, Archipelago Sea, Gulf of Riga and Estonian coastal waters (their more souther management unit). The status of ringed seals is not directly evaluated for these two management units in this assessment and no thresholds have been established. However, the nutritional status of ringed seals is suggested to be declining based on a statistical analysis of data from hunted seals (Kauhala et al in prep).

Harbour seals are confined to the Kalmarsund (Western Gotland Basin and Bornholm Basin), Western Baltic Sea (Arkona Basin, Bay of Mecklenburg, Kiel Bay, Great Belt, the Sound), the Kattegat and Limfjord; each of which are separate management units. The Kattegat and Limfjord subpopulations may be approaching carrying capacity since the annual growth rates are levelling off. Threshold values with regard to blubber thickness are not finally determined and no status evaluation is made in the current assessment.

 

The indicator structure and methodology is well defined for grey seal populations recovering from low population sizes. Further work to assess the grey seal population using data from a wider spatial region (i.e. more countries) would benefit the assessment confidence. Furthermore, additional work is needed to properly assess all species of seals that occur in the Baltic Sea and for those species/populations or management units/subpopulations approaching carrying capacity (i.e. when the population size is so large that density dependence sets in) the implementation of a suitable revised threshold to correspond to the ecological reality (e.g. food availability) is required.

Confidence of the indicator evaluation for grey seals is considered to be intermediate. The methodology is standardised and the grey seal population is highly migratory, travelling widely in the Baltic Sea region, however the sample sizes for the current assessment are somewhat low and limited in spatial coverage.

 

The indicator described here is applicable in all HELCOM sea regions since the grey seal population can be considered as a shared population. However it should be noted that due to migration patterns, fisheries activities and national hunting regulations, the shared seal population is exposed to differing pressures in each country. To balance for these issues an additional more detailed measure of seal health is being developed to evaluate the causes behind observed trends. A detailed Health Indicator would complement the current Nutritional Status indicator and become a powerful tool to evaluate the state of the environment.

The indicator is applicable in the waters of all the countries bordering the Baltic Sea since the indicator includes one or more marine mammal species that occur in all HELCOM assessment units.  In the current document only the grey seal nutritional status is directly assessed and the status evaluation for the entire Baltic Sea region is extrapolated from data gathered from Finland and Sweden. Improved data coverage and data calls/reporting is planned for future updates.

 

Relevance of the core indicator

Marine mammals are top predators in the marine ecosystem and therefore good indicators of changes in  biotic and abiotic environment, for example variation in food webs. Marine mammals accumulate fat soluble hazardous substances such as heavy metals and PCBs in their tissues (so called bioacumulation) and thus reflect the level of pollution in the environment. Seals are also affected by human disturbance such as hunting, fishing by-catch and disturbance (e.g. chemical and noise pollution), as well as infectious diseases and climate change.

Distributions of different species during feeding and annual migration encompass the entire Baltic Sea, although no land-based haul-out sites occur in Germany, Latvia and Lithuania. Monitoring of the nutritional status of seals occurs in all countries where data on stranded, by-caught or hunted seals are collected.

Blubber acts as the energy storage of seals and thus a reduction in blubber affects reproduction and survival of individual seals and is an early warning of decline in population trends, as confirmed by multiple scientific studies worldwide. Although blubber thickness responds to short-term variations in the environment and is a versatile indicator that complements the population trend and reproductive rate indicators, other relevant aspects of seal health for environmental monitoring are not captured by this indicator. For example, a sudden increase in pathological changes in the reproductive tract, an increase in intestinal wounds, or a new invasive parasite may also be important indicators of environmental disturbances, but are not captured properly by the current indicator alone. Therefore, a new Seal Health Indicator, indicative of wider population level trends and encompassing a wider range of potential causative factors, is required and is the current focus of further development work.

Temperature variations across the latitudinal extent of the Baltic Sea have been suggested to influence certain biological processes or community factors, however there is currently no documented evidence for a spatial variation in regulation of blubber thickness in sub adult seals from the genetically mixed populations due to temperature. The indicator methodology is thus deemed to be an informative approach in assessing the nutritional status of seals across the entire Baltic Sea region. It is therefore most likely that the average fat layer variations in grey seals between years represent changes in food availability and other stressors and not sea water temperature. Fat layers are built up during the autumn, and selection (i.e. increased mortality in lean seals) occurs after that; in February during the coldest months. Thus fat layer variation in the autumn samples cannot be a response to temperature in the winter to come. Adaptation to warmer water temperature in the long run will be a much more slow evolutionary process over many generations of seals.


Policy relevance of the core indicator

 BSAP segment and objectivesMSFD Descriptor and criteria
Primary link

Biodiversity

  • Viable populations of species

D1 Biodiversity

D1C3 Population demographic characteristics of the species are indicative of a healthy population which is not adversely affected due to anthropogenic pressures.

Secondary link

Biodiversity:

  • Thriving and balanced communities of plants and animals

Hazardous Substances

  • Healthy wildlife

D1 Biodiversity

D1C2: The population abundance of the species is not adversely affected due to anthropogenic pressures, such that its long-term viability is ensured.

D1C4: The species distributional range and, where relevant, pattern is in line with prevailing physiographic, geographic and climatic conditions. D4 Food-web

D4C4: Productivity of the trophic guild is not adversely affected due to anthropogenic pressures.

D8 Contaminants

D8C2: The health of species and the condition of habitats are not adversely affected due to contaminants including cumulative and synergetic effects.

Other relevant legislation: In some Contracting Parties also EU Water Framework Directive – Chemical quality, Habitats Directive.


Cite this indicator

HELCOM (2018) Nutritional status of marine mammals. HELCOM core indicator report. Online. [Date Viewed], [Web link].

ISSN 2343-2543

 

Download full indicator report

Nutritional status of seals HELCOM core indicator 2018 (pdf)